Physics – 3.2.2 Refraction of light | e-Consult

Internal Reflection: Internal reflection occurs when a light ray travels from a medium with a higher refractive index (n1) to a medium with a lower refractive index (n2) and strikes the interface between the two media at an angle greater than the critical angle. Instead of refracting (bending) through the interface, the light ray is reflected back into the higher refractive index medium. This happens because the angle of incidence exceeds the critical angle, and the refractive index difference causes the light to be entirely reflected.

Total Internal Reflection (TIR): Total internal reflection is a specific case of internal reflection. It occurs only when light travels from a denser medium (higher refractive index) to a rarer medium (lower refractive index) and the angle of incidence is greater than the critical angle. The critical angle (θc) is the angle of incidence at which the angle of refraction is 90 degrees. When the angle of incidence exceeds θc, all the light is reflected back into the denser medium. This is because the light is essentially 'trapped' within the denser medium.

Experimental Example: A common experiment to demonstrate internal reflection is using a laser pointer and a prism. Shine the laser pointer onto the prism at an angle. Observe the laser beam within the prism. If the angle of incidence is correct, the laser beam will undergo internal reflection and be guided along the prism's path. This demonstrates how light can be confined within a medium through repeated internal reflections.

Everyday Example: Optical fibres utilize total internal reflection to transmit light signals over long distances. Light is injected into one end of the fibre, and it travels along the fibre core due to repeated TIR. The core has a higher refractive index than the cladding, ensuring that light is reflected back into the core and doesn't escape. This allows for data transmission without significant signal loss.